Transparent materials have been widely used in a variety of articles, including glazing materials for buildings and vehicles, laboratory glassware, packaging, decorative lighting fixtures, safety panels at sports arenas, and architectural panels. Common transparent materials include glass and polymers, such as acrylic and polycarbonate.
Glass may be the most widely used of these materials. Glass is hard, chemically inert in the presence of most other substances, resistant to abrasion, and inexpensive. These qualities make glass useful in many applications. However, glass is also heavy, brittle and difficult to shape into complex forms. Standard sheet glass, for example, may have low tensile properties of about 1,000 psi. These qualities make glass unsuitable for some applications.
Polymers, such as acrylic and polycarbonate, may be lighter in weight, less brittle, and easier to form into complex shapes when compared to glass. Plastics, such as polycarbonate, are more resilient than glass, but they can only produce tensile strengths of about 10,000 psi. Polymers have been used extensively in many industries. For example, poly (methyl methacrylate) has found beneficial use in many products. Unfortunately, polymers may not possess the strength of glass. Materials comprising a combination of glass and polymer have been used in applications where neither material alone is desirable.
Laminate materials comprising layers of glass, acrylic or polycarbonate plastics, or combinations thereof bonded together by interlayers of a polymeric bonding material have been described. A glass/plastic windshield, for example, may comprise a glass face ply laminated to acrylic structural plies by means of polyvinyl butyral (PVB) interlayers. Unfortunately, bond failures, or delaminations, at the interlayer interfaces have been noted. Causes for bond failure may include mechanical or thermal stress, moisture ingress, and bond deterioration.
Other materials comprising a combination of glass and polymer have also been disclosed. A composite structure for use as pipes or storage tanks has been described as fiberglass fibers disposed in layers and impregnated in a resinous binder. To balance and distribute material strength in all directions, fibers of one layer are disposed at an angle relative to the fibers of a second layer. Although containment strength and resistance to delamination is increased, these materials may not be optically clear and may not be suitable for many applications.
A transparent material has been disclosed in U.S. Pat. No. 5,733,659. This molded material comprises a pair of thermoplastic films and a reinforcing resin composition layer interposed between the films. The reinforcing resin composition comprises a thermoplastic resin, such as an aromatic polycarbonate resin, a glass filler and polycaprolactone. The glass filler is in the form of beads, flakes, powder or chopped fiber strands and the filler is uniformly blended into the resin. The amount of glass filler is preferably 5 to 30% (about 2-18% by volume) by weight based on a total weight of the molded product. To produce this transparent material, the thermoplastic films are fitted on the inner surface of an injection mold and then the reinforcing resin composition is melt-injected into the injection mold. Although increasing the amount of glass filler increases the strength of the transparent material, it is also said to considerably deteriorate the optical properties. Additionally, a primer coating treatment may be necessary to promote a thermal fusion between the thermoplastic film and the reinforcing resin composition. Further, these processes may not be suitable when using fabric fillers or unchopped fiber strand fillers.
Other methods of producing transparent composites have been described by Day et al. in “Optically Transparent Composite Development,” Final Technical Report (Z10045), McDonnell Aircraft Company, 1992. Although composite strength increased with increasing filler content, this report also noted that optical transmission decreased. The transmission decease was said to be due to the large number of interfaces where transmission losses occurred. By placing fiber fillers where bending stresses are highest, the amount of filler required for a given flexural strength was reduced. Unfortunately, these composites may be transparent only over a narrow temperature range and may not possess the strength needed for some applications.
A laminate formed by polymerizing a monomer while glass fibers are maintained immersed within the monomer has been described in U.S. Pat. No. 5,039,566. These materials are said to be useful as aircraft canopies and aircraft windows. Matching the refractive indices of the glass and polymer minimizes the scattering and reflection of light that normally occurs at the glass/polymer interface. Unfortunately, the described processes require several hours or days and the optical clarity of the resulting material is inadequate for some applications. Further, as the percent glass volume increases, the optical clarity of the material decreases. In one sample, the percent glass volume was 14 and the optical transmission was 43.3%. In a second sample, the percent glass volume was 33 and the optical transmission was about 20%. Although the first sample had higher optical transmission, it also had a modulus of rupture of only 47,796 lbs./in2, while the second sample had a modulus of rupture of about 90,000 lbs./in2. Additionally, these materials may have acceptable optical clarity only when the material is exposed to a narrow range of temperatures and they may be difficult to form into complex shapes. Further, the manufacturing processes provided may not be desirable for many applications and may require secondary installation steps.
Some of the disadvantages of the '566 patent were addressed in U.S. Pat. No. 5,665,450. For these composites, four-sided glass ribbons, as opposed to cylindrical fibers, were embedded in a polymer sheet. The ribbons were arranged in such a way that one planar surface of the ribbon was parallel to the surface of the polymer sheet. This composite sheet was then laminated onto one or both surfaces of a polymer core. By positioning the glass near the surface, the strength of the material was said to increase without also increasing glass volume and without decreasing the optical transmission. The glass volume was preferably between 1% and 25% of the composite volume. Materials containing ribbons were found to have a higher transmission over a wider temperature range than those containing fibers. Unfortunately, optical transmission over still wider temperature ranges is needed. Also, processes that require the careful arrangement of ribbon surfaces parallel to the surface of the polymer sheet may not be preferred when high volume laminate production is desired. Further, it was noted that a higher maximum transmission was observed when using the cylindrical fibers. Additionally, because the composite sheets were laminated onto pre-formed polymer cores, these materials may not be preferred when complex component shapes are desired. Also, the production of these materials may require high pressure. Although, these materials may have improved optical properties when compared to the '566 patent, further increases in laminate strength are needed.
As can be seen, there is a need for improved optically clear structural laminates. Further, there is a need for laminates having improved optical properties without a reduction in percent glass volume. Additionally, there is a need for transparent laminates having decreased weight and increased structural performance. Also, there is a need for laminates having increased tensile strength and modulus (increased stiffness), and increased optical clarity. Further, there is a need for optically clear laminates capable of being easily formed into complex shapes. Moreover, there is a need for laminates having improved optical properties over a range of temperatures without the need for four-sided ribbons.